The overall research goal of this proposal is to investigate the potential of fluid phase endocytosis (pinocytosis) as a strategy for delivering diagnostic drug carriers efficiently into tumor cells. Accelerated fluid phase endocytosis is a prominent feature of rapidly proliferating cells, hence always present in neoplasia. The approach potentially has advantages over "tumor-recognizing", target specific probes since it is applicable to a wide variety of tumors and results in cellular internalization. The proposal addresses two specific aims: 1) to investigate the mechanism and regulation of fluid phase endocytosis in tumor cells and 2) to develop methods for in vivo imaging of tumor cell endocytosis (TCE) using animal models of cancer. We have previously observed that long circulating macromolecules such as Protected Graft Copolymers (PGC), obtained by derivatization of polyaminoacids with methoxy poly (ethylene glycol) esters, are efficiently internalized into tumor cells in culture (0.8-4.1 x 106 molecules/cell) in a cell cycle dependent manner. Preliminary studies suggest that the cellular uptake of these long-circulating macromolecules is mediated by fluid phase endocytosis. One major aspect of the proposed research is to determine the cellular mechanism governing and regulating this pathway of PGC uptake. More specifically, we will: 1) investigate the kinetics of cellular uptake; 2)attempt to elucidate the mechanism of cell-cycle dependent activation of TCE; 3) determine pathways of intracellular sorting; and 4) investigate compartmentalization and exocytosis of PGC in tumor cells and in phagocytes, as these processes are relevant to subsequent imaging research. In addition of TCE studies in vitro we have demonstrated for the first time that PGC probes also accumulate in tumors in vivo at significant levels. In the current proposal we will further quantify fractional tumor accumulation (interstitial and intracellular) in human xenograft tumors and determine the heterogeneity of such tumoral distribution. We will specifically test the hypothesis that actively proliferating tumor cells (esp. perivascular cells) actively engage in TCE of long- circulating probes. Furthermore, we propose to address clinically relevant issues of TCE imaging, e.g. tumor detection. Another aspect of the proposed research is related to MR imaging and addresses the issue of tumor growth mapping in vivo. The long term goal of this research is to develop methods and probes for in vivo imaging of tumor physiology as it is relevant for detection, typing and therapy of cancer.